Thermoplastic materials or so-called “hot melt” materials such as adhesives which are used for various coating and bonding operations are usually stored in solid or particulate form and must be converted to the molten state before they can be supplied to dispensers. Thermoplastic processing systems typically used in commercial applications involve melting the material in a large melting unit so that the material is converted to a liquid or flowable material, transporting it at high pressure through one or more heated hoses over considerable distances, and then distributing it to one or more dispensers which apply the liquid adhesive to a substrate material. For the purposes of the present disclosure the terms melt, liquefy and make flowable are referred to as rendering a material able to flow according to desired characteristics. These melting units generally include (1) a hopper (also referred to as a storage vessel or tank) having an opening for receiving solid thermoplastic material, (2) several heating elements mounted within the tank used primarily to convert the solid thermoplastic material to the molten state, (3) a reservoir and/or manifold for receiving the molten material, and (4) a pump for pressurizing and transporting the molten material to the manifold and ultimately off to one or more dispensers. One popular variation to this design is the grid type melter which adds a grid unit that consists of extended fins located within the hopper or tank, and mounted above the reservoir for the purposes of improving the melting capability of the unit. While there have been minor improvements to such systems the basic design architecture has remained the same for many years.
The current system architecture and apparatus designs have many inherent operational shortcomings, which largely come from the use of relatively large often open heated tanks to melt the thermoplastic material and the necessity to transport the molten material over great distances using hoses and/or pipes. The extensive amount of uncovered heated surfaces and pool of molten material inside the tank inherently expose users to serious burn hazards. Also potentially dangerous is the requirement of high pressures to transport the material between the melting unit and the dispensers through heated hoses. The large mass of the tank and its related components necessitate long warm-up time periods resulting in extended system downtimes thus decreasing the productivity of manufacturing lines. Additionally, the large surface area of the tank and its related components allow extensive heat losses resulting in a highly inefficient system that wastes significant energy. Design attributes typical of current systems expose the molten adhesive to air, hold the adhesive at high temperature for long durations, recirculate the adhesive multiple times through the system, and trap adhesive in non-flow areas. These all combine to accelerate the degradation of the thermoplastic material. This often results in charring, which can lodge in the small openings of the dispensing nozzles and cause it to clog unexpectedly. Such clogs are a major source of system failure and system downtime. Furthermore, flexible heated hoses, which are commonly used to transport the molten adhesive from the tank to the dispenser, expand and contract with changing hydraulic pressure thus creating an undesirable system capacitance. Consequently, as the line changes speeds the hose volume changes thus causing inaccurate output flow and therefore variances in material deposition. Finally, the current systems require many and unnecessary large and complex components which make it difficult to locate and install on equipment that has limited space constraints.
It is therefore desirable to have a system that removes many of the safety hazards inherent with current systems, warms up very quickly, eliminates major design attributes that lead to material degradation and thus materially improves system reliability, improves delivery accuracy, consumes significantly less energy, decreases the number of required system components, and reduces the overall size and complexity of the system.